diff --git a/opt/bignum.c b/opt/bignum.c
index 835b354f..61bde456 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -225,8 +225,10 @@ sexp sexp_read_bignum (sexp ctx, sexp in, sexp_uint_t init,
   if (c=='.' || c=='e' || c=='E') {
     if (base != 10)
       res = sexp_read_error(ctx, "found non-base 10 float", SEXP_NULL, in);
-    if (c!='.') sexp_push_char(ctx, c, in);
-    res = sexp_read_float_tail(ctx, in, sexp_bignum_to_double(res), (sign==-1));
+    else {
+      if (c!='.') sexp_push_char(ctx, c, in); /* push the e back */
+      res = sexp_read_float_tail(ctx, in, sexp_bignum_to_double(res), (sign==-1));
+    }
   } else if ((c!=EOF) && ! is_separator(c)) {
     res = sexp_read_error(ctx, "invalid numeric syntax",
                           sexp_make_character(c), in);